1. Field of the Invention
This invention relates to methods for reformulating contaminated, mixed plastic wastes to achieve a reclaimed plastic feed material that has properties suitable for use in new products, thereby increasing the value of the resultant recycled plastics. The reformulated materials are desirably produced by preprocessing and analyzing various batches of mixed waste plastics as disclosed herein, and then blending portions of the various batches, each having different plastic contents, physical properties and contaminant levels, to produce reclaimed feed materials for new products. Depending upon the reclaimed wastes that are available and the desired properties of the reformulated mixed waste plastics, the blends can include either reclaimed materials alone, or reclaimed materials in combination with virgin resin. This invention is particularly useful for reformulating mixed waste plastics comprising a significant portion of plastic film.
Another aspect of the invention relates to technology and methods for characterizing and identifying various types of polyethylene that are present, sometimes with polypropylene, in mixed waste plastics from which most of the other polymeric and non-polymeric contaminants have been removed. A further aspect of the invention relates to methods for analyzing and blending mixed waste plastics by taking into consideration the plastic content, density, rheology, filler content, type and level of other contamination, and color or pigmentation.
2. Description of Related Art
The need to reclaim, recycle and reuse waste plastics is well known and is currently at the forefront of public, private and governmental interest. Most plastics are manufactured from petrochemicals, and the current demand for petroleum is at an all-time high in the United States and elsewhere throughout the world. Additionally, many environmental concerns are associated with the handling of waste plastics, primarily due to their slow degradation using conventional waste treatment methods and due to the hazards they present for wildlife.
Mixed waste plastics, and especially those obtained from municipalities, typically comprise many different types of contaminants that must be removed or otherwise dealt with in any effective plastic reclamation process. Such contaminants can include, for example, non-melting fillers, pigments, wood, paper or metal, as well as a variety of plastics that may not be suitable for use as a feed material for the products or process under consideration. Various plastics that may be present in mixed waste plastics, depending upon the source, can include, for example, polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), ethylene-vinyl acetate (EVA), polyvinylidene chloride (saran), acrylonitrile butadiene styrene (ABS), and the like.
Even within a particular plastic category such as polyethylene, for example, there are different analogs such as low density (LDPE), linear low density (LLDPE), and high density (HDPE) polyethylene that may have similar densities but functional characteristics that differ significantly. In many cases, reclaimed plastics are manually sorted to segregate the different types of plastics prior to recycling. This can be a very expensive and labor intensive process, and mixed waste plastics are sometimes transported overseas for sorting and classification to permit reuse. Even where the waste originates from a common source and is represented to have substantially the same content from batch to batch, variations in the feed material can require reactive adjustments in manufacturing procedures that cause production losses through increased downtime and substandard products.
The ability to use a higher percentage of mixed waste plastics in the manufacture of new products, including but not limited to composite wood and plastic building materials, is highly desirable. Although many products have been manufactured successfully using scrap or recycled plastics of various types, the variability that exists in the composition and cleanliness of batches of mixed waste plastics obtained over time from either the same or different sources has previously caused serious problems with raw materials processing and manufacturing. The reclamation and reuse of polyethylene film is particularly problematic. In 2005, the U.S. Environmental Protection Agency reported (EPAS530-R-06-011) that less than 3% of all polyethylene film was recycled. Consequently, millions of tons per year of polyethylene film is buried in landfills and never reused. Such film can include, for example, trash bags, shopping bags, bubble wrap, shrink wrap, meat packing wrap, blood bags, and nursery and greenhouse films.
Another problem has recently arisen in the reuse of reclaimed mixed plastics due to the higher filler content of such plastics. As the price of oil and virgin plastic resins rise, plastics manufacturers are incorporating a higher percentage of mineral fillers in the plastics. Consequently, many plastic bags now contain from 15-20% calcium carbonate filler, and widespread filler loadings of up to 30% are predicted. Filler contents can even be pushed as high as 40%, 50% and even 60% where more refined (and expensive) filler particles are used. The increased use of fillers in plastics necessarily impacts the physical properties of those plastics, the associated products, and ability to effectively recycle the resultant mixed waste plastics.
Various analytical methods have been used with in the past to determine the types and properties of plastic present in mixed waste plastics, but with limited success. For example, batches of mixed, reclaimed plastics have been analyzed by pressing a sample of the material between two hot plates at a suitable temperature to form a test plaque, which is then cut up and repressed several more times to make it more homogeneous. Sometimes the polymers present in such test plaques can be determined by visual inspection, although this method is highly inaccurate and only allows for gross distinctions to be made. At other times the samples may contain contaminant inclusions that are not representative of the entire batch and thus can skew the analysis. Other methods believed to have been tried to characterize mixed reclaimed plastics include, for example, melt filtration and solvent extraction. Some prior art patents disclosing inventions relating to reclaiming and reprocessing plastics are, for example, U.S. Pat. Nos. 4,014,462; 4,038,219; 4,071,479; 4,187,352; 4,225,640; 4,379,525; 4,968,463; 5,075,057; 5,217,655; 5,503,788; 5,618,881; 5,888,910; 5,914,353; 5,967,435; 6,107,400; 6,228,479; 6,262,133; 6,797,216; 6,849,215 and 6,984,694.
More recently, the use of differential scanning calorimeters (DSCs) for various purposes has been further described and explained, for example, in various papers published by Perkin Elmer, including the following: Use of Differential Scanning calorimetry in Testing of End-Use Characteristics of Thermoplastics; Prediction of End-Use Characteristics of Polyethylene Materials Using Differential Scanning calorimetry; Application of DSC to Injection Molding; DSC Isothermal Crystallization Studies for Better Injection Molding of Polymers; and DSC as Problem Solving Tool: Characterization of Consistency of PFA Resins.
Notwithstanding recent advances known to those skilled in the art, a need still exists for a reliable and efficient method for characterizing the properties of multiple batches of mixed plastic wastes of varying compositions and for combining them in the amounts needed to produce substantially homogeneous blends of mixed plastics exhibiting specified physical properties within predetermined ranges that are known to be acceptable for use in manufacturing products having desired characteristics.